1. Field of the Invention
The invention relates to a method for determining properties of three-dimensional objects that were produced using a thermal generative method, particularly using a thermal generative method, for example by means of laser sintering, FDM (Fused Deposition Modeling), or SMS (Selective Mask Sintering).
2. The Prior Art
“Rapid technologies” are used for numerous technical fields. Technologies in this regard, using stereolithography on the basis of photopolymerization, are known from WO 92/11 577 A 1 and DE 44 14 775 A 1. In this connection, three-dimensional objects are produced by means of locally limited chemical reactions, in that liquid monomers or oligomers are crosslinked to produce a solid polymer, by means of applying UV radiation. Buildup of these three-dimensional objects takes place in layers. As soon as a layer has been produced photochemically and polymerized, new liquid material is applied to this layer, and also polymerized afterwards, so that another layer is formed.
In laser sintering, a 3D CAD file is transformed into a 2D file, by means of cutting it into thin layers having a thickness of typically 0.10 mm to 0.15 mm, and afterwards transmitted to the process computer. This computer controls the IR radiation that scans the surface of the powder layer, depending on the contour of the 2D data, by means of a system of optical lenses and mirrors. In this connection, the powder is sintered in a layer, with point precision. After a layer has been processed, the construction platform is lowered in accordance with the selected layer thickness, and a new layer of powder is applied. Thus, a three-dimensional object is generated layer by layer. A system in this regard is known, for example, from DE 10 2004 012 682 A 1.
Aside from laser sintering, the technical solutions called FDM (Fused Deposition Modeling) and SMS (Selective Mask Sintering) are also increasingly being used as thermal generative methods.
Independent of the concrete embodiment of method sequence and device technology, such thermal generative methods have fundamentally proven themselves. After the use of these methods was extensively restricted to the production of prototypes during the development process, at first, this technique is also increasingly being used for mass production of products in small and medium piece numbers. Thus, shorter development times, faster production introduction times (“time to market”) and often better quality of the products in question can be achieved, in comparison with conventional methods.
Despite these undisputed advantages, the use of thermal generative methods is problematic, at least for mass production of safety-relevant components. Components for machines, systems, land vehicles, or aircraft must withstand greatly varying stresses. The usability and useful lifetime of these components is often limited by process-related variation in density or by the occurrence of cracks at critical locations of the construction, which depend on numerous influence factors, such as, for example                unforeseeable or uncontrollable process variations,        geometric peculiarities of the components,        material variations when changing lots or due to changes in climatic conditions,        overload due to mechanical, thermal and/or chemical stresses, especially of the surfaces.        
This mutual dependence and the influences that cannot be precisely foreseen lead to greatly different prognoses concerning the ability to withstand stress and the useful lifetime of the components that can actually be achieved. It is true that such components can be tested with regard to density and strength by means of measurements regarding the geometrical parameters and by means of weight determination. However, smaller defects within a layer or within a small component area are not detected with such tests.
In this connection, studies by the applicant have shown that in the case of thermal generative methods (e.g. laser sintering system with a temperature of 175° C. in the work area), even slight temperature differences of 1° K to 3° K bring about differences in density on the order of 5% to 10%. With reference to the aforementioned working temperature, deviations in the process parameters of only 1% to 2% will therefore result in density differences of up to 10%. If it is furthermore taken into consideration that a density that differs by about 10% can cause a strength loss of up to 50%, it becomes obvious that there is a significant demand for the development of a method for destruction-free determination of typical material parameters of three-dimensional objects, with which method even small structural defects can be determined. However, such a method has not become available up to the present. Even a transfer of test methods in other technical fields to the thermal generative methods that might be considered by a person skilled in the art does not yield any suitable solution approach.
For example, DE 198 46 325 A 1 describes a technical solution for determining the effectiveness of sterilization of medical technology equipment using a chemo-indicator. This chemical substance brings about a change in color after a minimum action time of the necessary process parameters, whereby the end point of the color change is defined and described by the manufacturer. Accordingly, a statement concerning the effectiveness of the sterilization process is possible by means of a visual assessment of the indicator.
A device for optical evaluation of colorimetric discoloration zones on a carrier, for detection of components of a gas mixture in gas or vapor form, is known from DE 43 03 858 A 1. In this in-process inspection, light is transmitted through discoloration zones, which light either does or does not trigger a signal, by way of photocells. As a result, a statement concerning the progress of the discoloration zones when an established limit value is reached is achieved on the basis of the time sequence of these individual signals.
Although the aforementioned DE 198 46 325 A 1 and DE 43 03 858 A 1 have an interesting departure point, in that destruction-free component testing is implemented by means of evaluating color changes, these technical solutions are not suitable for thermal generative methods, in which significantly different material, temperature, and pressure conditions prevail. Furthermore, these methods only allow a general statement about whether or not a previously defined method point has been reached, in any case, but they do not allow the determination of specific material parameters.